Currently, in our country Japan, various edible oils have been used in large amounts. A part of used oil (waste edible oil) is reused as a raw material for soap or the like. However, a majority of the used oil are not collected, and are actually transported to refuse disposal, and incinerated together with burnable refuse, or reclaimed together with unburnable refuse.
On the other hand, there has been previously known that an alkyl ester of a fatty acid can be prepared by subjecting a monoglyceride, a diglyceride or a triglyceride, which is a main component in a vegetable oil, to transesterification reaction with an alkyl alcohol, thereby giving an alkyl ester of a fatty acid (for instance, “Yuki Kagaku Handobukku (Organic Chemistry Handbook),” published by Gihodo, 1988, p. 1407-1409). Also, by utilizing this reaction, various studies have been made on the techniques for preparing an alkyl ester that can be used as a diesel fuel oil from a vegetable fat or oil, a waste edible oil or the like (for instance, Japanese Patent Laid-Open Nos. Hei 7-197047, Hei 7-310090 and the like). In these techniques, alkyl esters which can meet the requirement of the current Quality Assurance Regulation regarding a gas oil have not been obtained.
Incidentally, since the transesterification reaction is an equilibrium reaction, the equilibrium is shifted to the product side by using an alkyl alcohol, one of the raw materials, in a large amount, or removing glycerol produced as a side reaction product, thereby increasing an yield. Also, this reaction is said to be more advantageous in a vapor phase reaction than in a liquid phase reaction from the viewpoint of state of equilibrium. Further, in order to increase the reaction rate, a catalyst is generally utilized.
In the preparation of acetic acid, a higher fatty acid, an unsaturated carboxylic acid or the like, which is a representative industrial process of the transesterification reaction, an acidic catalyst is generally used in a large amount. For instance, a protonic acid such as sulfuric acid or phosphoric acid has been used as an esterification catalyst for non-aromatic carboxylic acids, and boric acid or sulfuric acid has been used for an esterification of a phenolic acid. However, since these reactions are basically a homogeneous reaction system in which a catalyst exists in the dissolved state in a reaction solution, there has been a problem that the separation and collection of the catalyst from the formed liquid are difficult.
A solid acidic catalyst is also well used. In the transesterification reaction of terephthalic acid or methacrylic acid, there has been used SO42−—TiO2, TiO2—SiO2, Al2(SO4)3/SiO2.Al2O3, a sulfonic acid-based ion-exchange resin and the like. In addition, a heteropoly-acid is said to be an excellent esterification catalyst. In a case where the heteropoly-acid is supported to SiO2 or activated charcoal, the heteropoly-acid as a vapor phase catalyst has been known to exhibit an activity which is higher than those of SiO2—Al2O3 and the solid phosphoric acid. Further, a clay mineral has been also used as a catalyst. Since these solid acidic catalysts and mineral catalysts do not have to be separated from the formed liquid, use of these catalysts is more excellent from the viewpoint of simplification of the reactor. However, these industrial catalysts have a fatal defect that the activity for the transesterification reaction of a fat or oil is low. Therefore, the above process has not yet been actually used on an industrial scale to date.
As a technique for applying a solid acidic catalyst to transesterification of a fat or oil, there has been proposed a technique as disclosed in, for instance, Japanese Patent Laid-Open No. Hei 6-313188. Moreover, the catalyst used in this technique includes a simple or composite metal oxide, a metal sulfate, a metal phosphate, an immobilized acid in which the acid is supported or immobilized to a carrier, a natural mineral and a layered compound, a solid heteropoly-acid, a superacid, a synthetic zeolite, an ion-exchange resin and the like. However, in this technique, the catalytic activity for the transesterification reaction of a fat or oil is as low as that of the conventional processes described above. Therefore, in order to achieve a high yield, there has been necessitated that a ratio of the solid acidic catalyst is increased in the reaction system or that the reaction time is lengthened.
A basic catalyst has been also used in the transesterification reaction, and there has been known that a metal alcoholate is effective as this basic catalyst. Therefore, sodium alcoholate or potassium alcoholate has been generally used as the metal alcoholate. Also, as the basic catalyst, sodium hydroxide, potassium hydroxide, sodium carbonate or the like has been used. These exhibit high activity for the transesterification reaction of a fat or oil. However, the conventional basic catalyst acts in a dissolved state in a reaction solution in the same manner as the acidic catalyst mentioned above. Therefore, the basic catalyst dissolves in the formed liquid, so that the problem that its separation and collection are difficult has not been eliminated.
In addition, there has been attempted to use a solid basic catalyst in the transesterification reaction, and as such a solid basic catalyst, there has been proposed an ion-exchange resin having an amine-based base (for instance, Japanese Patent Laid-Open No. Sho 62-218495). In this technique, the problem of separating and collecting the catalyst is not basically generated. However, this technique is carried out in a reaction system in which the alcohol is used in excess and the concentration calculated as triglyceride, is 0.1 to 3% by weight or so, so that the activity is drastically low, the reaction temperature is also limited to 60° C. or lower from the viewpoint of the durability of the ion-exchange resin, and the like. Therefore, it cannot be said to be practical.
Also, recently, use of a basic solid catalyst comprising a carboxylic acid compound and iron oxide, or a potassium compound and zirconium oxide has been disclosed (Japanese Patent Laid-Open No. 2000-44984). However, its catalytic activity cannot be said to be satisfactory, thereby making it impractical.
In addition, in a case where the basic catalyst is used, other problems are generated besides the problems as mentioned above. In other words, since a natural fat or oil generally contains a large amount of free fatty acids (3% by weight or more on average), if the basic catalyst is used, the side reaction of the formation of the fatty acid soap is drastic, so that there arise some problems that the catalyst is required in excess, that the separation of the fatty acid ester layer and the glycerol layer becomes difficult due to the generated fatty acid soap and the like. Therefore, a pretreatment step of removing a free fatty acid would be necessitated.
From the viewpoint of avoiding the above problems, there has been disclosed, for instance, in Japanese Patent Laid-Open No. Sho 61-14044, a process for converting a free fatty acid to an ester with an acid catalyst as a pretreatment step. However, there arise some problems that the separation of the acid catalyst is difficult, and if the acid catalyst remains in the reaction mixture when the transesterification reaction is carried out, the basic catalyst (a metallic alkali catalyst) is undesirably neutralized, and thereby the amount of the solid catalyst used is increased by the amount of the neutralized catalyst.
In addition, as a process for preparing an ester of a fatty acid not necessitating the pretreatment step as mentioned above, there has been proposed a process of using a solid acidic catalyst (for instance, Japanese Patent Laid-Open No. Hei 6-313188). However, the acidic catalyst has a fatal defect that the activity for the transesterification reaction of a fat or oil is low as compared to that of the basic catalyst, so that there arise a problem that the catalyst is required to be used in a large amount in the transesterification reaction in which the acidic catalyst is used.
Furthermore, there has been known a technique of carrying out the transesterification reaction under high-temperature and high-pressure conditions (240° C., 9 MPa) in the presence of a basic catalyst, thereby increasing the reaction efficiency without requiring the pretreatment. Such a technique is described, for instance, in “JAOCS” (Vol. 61, No. 2, p. 343, 1984). However, since a homogeneously-used catalyst is used, there has been necessitated a purification step as an after-treatment such as removal of the catalyst or removal of the fatty acid soap partially produced.
Moreover, as a process for preparing an alkyl ester of a fatty acid, there has been disclosed a process of carrying out transesterification without a catalyst in an atmosphere in which an alcohol is made into a supercritical state (Japanese Patent Laid-Open Nos. 2000-109883 and 2001-143586). However, its reaction rate is notably smaller as compared to that of the homogeneously-used basic catalyst, and the reaction ratio at an equilibrium reached and the reaction rate are low under the conditions near the supercritical point, thereby making it almost impractical. In order to improve the reaction rate, the conditions of high-temperature and high-pressure must be made stricter. However, there are some problems that the decomposition of the reaction product takes place and that the reaction ratio is consequently lowered, and the like.